An EEPROM (Electrical Erasable and Programmable Read Only Memory) has been widely used as an electrically writable and erasable nonvolatile semiconductor storage device. A memory unit of such a nonvolatile semiconductor storage device represented by a flash memory is composed of a plurality of nonvolatile memory cells, and for example, a field-effect transistor that has a conductive floating gate electrode or a trap insulating film surrounded by oxide films below a gate electrode and stores information by holding electric charge in the floating gate electrode or the trap insulating film is used in this nonvolatile memory cell. A memory holding state of the floating gate electrode or the trap insulating film can be read out by the change of a threshold voltage of the field-effect transistor.
The trap insulating film is referred to as an insulating film capable of holding electric charge, and for example, a silicon nitride film can be cited as an example. The nonvolatile memory cell using a silicon nitride film as a charge storage film is referred to as a MONOS memory cell, and since it discretely stores the electric charge, the reliability of holding information is excellent as compared with the nonvolatile memory cell that stores the electric charge in the floating gate electrode. Also, in the MONOS memory cell, it is necessary to form an oxide film above and below the silicon nitride film in order to suppress the leakage of held electric charge. However, since the reliability of holding information is excellent, the oxide films can be made thin. For example, a thin oxide film of 8 nm or less can be applied, so that low voltage writing and erasing operations can be achieved.
However, in the MONOS memory cell using a silicon nitride film for the charge storage film, it is difficult to say that both of a charge capture characteristic and a charge holding characteristic are sufficient, and for example, the MONOS memory cell using a metal oxide film such as alumina instead of a silicon nitride film for the charge storage film has been proposed.
For example, the nonvolatile semiconductor memory cell in which the metal oxide film such as alumina is applied to the charge storage film in order to improve the charge holding characteristic is disclosed in Japanese Patent Application Laid-Open Publication No. 2004-158810 (Patent Document 1). Also, a technology of improving a dielectric constant of a metal oxide film by applying heat treatment to the film immediately after the metal oxide film is formed is disclosed in the specification of US Patent application publication No. 2006/0022252 (Patent Document 2). This heat treatment is generally performed after the metal oxide film is formed. Further, a technology in which a laminated film of a silicon oxide film and a silicon nitride film (a laminated structure in which a silicon nitride film is stacked on a silicon oxide film) is formed between a substrate and a metal oxide film in order to speed up the erasing operation is proposed (see Technical Digest of IEDM, 2006, p. 971 (Non-Patent Document 1)). Furthermore, a technology in which a device operation window is expanded by using a laminated structure of an alumina film and silicon nitride films (bottom silicon nitride film/alumina film/top silicon nitride film) for a charge storage film is proposed (see VLSI Technology Digest, 2007, p. 138 (Non-Patent Document 2)).